An RFQ accelerator is capable of accelerating an ion beam with a focusing force and thus capable of accelerating an ion beam of large current without divergence, and therefore is used as a high energy ion acceleration tube of a high energy ion implanter. The RFQ accelerator also has its application as an ion accelerator of experimental, analytical, and medical use.
As representative accelerators of charged particles, circular accelerator and linear accelerator are available. The circular accelerator, such as cyclotron, accelerates a beam in a circular motion and the linear accelerator accelerates a beam in a linear motion. The RFQ accelerator is an example of the latter. The linear accelerator works under the principle that ions are accelerated by application of a DC (Direct Current) high voltage between hollow electrodes. In this case, when the acceleration energy is qV, a DC power source capable of generating a high voltage V is required. Thus, in order to realize acceleration of several MeV, which is required in high energy ion implanters, a high voltage power source in the order of MV is required, and the power source section alone takes up a large space. Also, in such an accelerator for carrying out high energy acceleration, a vacuum chamber for passing a beam is required to have a large volume. As a result, the high energy accelerator such as above is inevitably large and expensive.
Meanwhile, in recent years, a demand for ion implantation with a high energy of several MeV has been increasing in semiconductor industry. However, to realize production in an industrial setting, a large device, which takes no account of costs, fails to meet such a demand, and there is a need for new and smaller accelerator capable of high energy acceleration.
As an accelerator suitable for such purpose, an RFQ accelerator has been getting an attention. The RFQ accelerator is one of relatively newer linear accelerators, and has a schematic arrangement wherein four electrodes are placed on a position corresponding to vertices of a square, and the electrodes on a diagonal line are connected to each other, and a radio-frequency voltage is induced between adjacent electrodes.
Namely, the four electrodes constitute a quadrupole, and a radio-frequency is applied between adjacent electrodes. Instead of applying a DC high voltage between electrodes which are separated from one another in a beam propagation direction, radio-frequency is induced between four electrodes parallel to the beam propagation direction. The radio-frequency is applied to quadrupole electrodes in this manner, thus the name RFQ, which stands for Radio-Frequency Quadrupole.
The RFQ accelerator was first proposed by Kapchinskii and Teplyakov (I. Kapchinskii and V. Teplyakov Prib. Tekh, Eksp.2 (1970) p.19). Then, it was first confirmed in 1981 that the RFQ accelerator is actually capable of carrying out acceleration in the Los Alamos National Laboratory of the United States (J. E. Stovall, K. R. Crandall and R. W. Hamm, IEEE Trans. Nucl. Sci, NS-28 (1981) P.1508).
Such an RFQ accelerator has a schematic structure wherein four electrodes (for example, A, B, C, and D in counterclockwise direction) are placed on a position corresponding to vertices of a square on a plane perpendicular to a beam propagation direction (z direction). On each of the four electrode rods are formed crest and trough portions in the lengthwise direction, and the electrodes are oriented such that the crest portions of a pair of electrodes, for example, electrodes A and C, correspond to the trough portions of the adjacent other pair of electrodes B and D, and that the trough portions of the pair of electrodes A and C correspond to the crest portions of the other pair of electrodes B and D. By inducing a radio-frequency voltage between each pair of electrodes A and C and the electrodes B and D, an accelerating electric field is generated in the beam propagation direction and a converging electric field is generated in a direction perpendicular to the beam propagation direction. A period between the crest and the trough of the electrode is called a cell.
Then, the time w/v in which ions travel over a distance w of a cell is set to be equal to a half-period T/2 of the radio-frequency. Namely, when the wavelength of the radio-frequency is .lambda., the distance w=vT/2=(v/c) (cT/2)=.beta..lambda./2. When the distance between adjacent crests is determined in this manner, ions pass through a cell per alternation of the accelerating electric field in the z direction. Thus, ions are accelerated by being subjected to electric field per cell. The RFQ accelerator functions as a linear accelerator because the propagation of ions and the alternation of the radio-frequency are synchronized in this manner. As ions are accelerated, v increases, and accordingly .beta.=v/c is also increased. Thus, the electrodes are designed such that the cell length increases progressively by small increments along the lengthwise direction of the electrodes.
As described above, the RFQ accelerator accelerates ions under the principle that is completely different from that of the conventional linear accelerator in which ions are accelerated linearly by application of a DC high voltage between electrodes which are separated from one another in the beam propagation direction. Thus, even through the RFQ accelerator is categorized as a linear accelerator by the fact that ions are accelerated in a straight line trajectory, the RFQ accelerator is largely different from the conventional linear accelerator in the arrangement of the electrodes and in the acceleration voltage, for which radio-frequency is used instead of direct current.
The RFQ accelerator has various advantages. First, it is not required to provide a large power source of a DC high voltage, instead a small radio-frequency power source is provided, thus reducing the volume of the power source section.
Secondly, the dimensions of the acceleration tube can be made compact. The cell length of the four electrodes is very small, and beam bore radius R.sub.1 is 4 mm. Thus, because the gap between the electrodes is narrow and the dimension in a direction perpendicular to the beam propagation direction is small, the cylindrical vacuum chamber surrounding the electrodes can be made sufficiently compact with a diameter of, for example, 600 mm. Further, the length in the direction of beam axis can be made short. For example, the length of the chamber is from 1 m to 3 m.
Thus, the RFQ accelerator is highly appealing in view of the power source requirement and the size of a vacuum chamber, and unlike the conventional linear accelerator of DC type, has a potential of realizing a practical accelerator in an industrial setting, such as manufacturing of semiconductors.
In the RFQ accelerator having the described arrangement, the present invention concerns the configuration of the four RFQ electrodes of A, B, C, and D, and their proportional relations to one another. The electrodes A, B, C, and D are provided extending in the beam propagation direction, and are rods each having crest and trough portions which are 180.degree. off-phase between adjacent electrodes (A and B, B and C, C and D, and D and A). Several points on the electrodes A, B, C, and D are supported by components called posts.
Posts provide a mechanical support of the electrodes A, B, C, and D to the inner wall of a tank (vacuum chamber), and form a resonance circuit in the tank. The electrodes A, B, C, and D and the posts generate a large amount of heat as a result of a large amount of radio-frequency current flowing on them. Thus, the electrodes A, B, C, and D are made of material having high electric and thermal conductivity, and a coolant is flown therein. In order to allow sufficient flow of a coolant, a coolant channel having a sufficient cross sectional area is required.
In the early stage of RFQ development, the ion beam had been accelerated in a low duty mode because of the heat problem. The duty is defined as the ratio of the time in which the ion beam is accelerated to a period of radio frequency. However, there has been a strong demand for an ion beam of large current, and there is a need to increase a duty. To realize this in an RFQ, a continuous wave (Cw) operation by means of increased cooling efficiency is needed.
An RFQ electrode which was first manufactured is a round rod having a waveform, as shown in FIG. 1. Since four rod electrodes provided are the same, the structure of only one electrode is shown. The entire periphery of a metal rod (copper, or alminium or iron plated with copper) having a circular cross sectional area is machined to have a waveform which is determined by the type of accelerating ion, input ion energy, output energy, and other factors, and a cavity for allowing a coolant to flow is provided inside the metal rod. It is possible alternatively to form a waveform by machining a metal material which has already been provided with a cavity. Such a waveform can be formed with ease, for example, by rotating a round rod, which is axially symmetrical, on a lathe. This technique has an advantage that the manufacturing is easy. Also, since the rod is axially symmetrical, it can be mounted on a post with ease for direction is not of concern.
As described, the early RFQ electrodes were solely for research purposes in laboratories and were for accelerating an ion beam of small current with low duty in a pulse operating mode. Accordingly, only a small amount of coolant was required. However, when it comes to high duty operation, the coolant channel of the electrode of FIG. 1 is too narrow to be applied for such a purpose, arising from the fact that the coolant cavity cannot be increased. Also, since the round rod is shaped into a waveform, the waist trough portions are weak and susceptible to bending. Further, mechanical oscillation is induced by the flow of a coolant. In particular, when oscillation is generated in the diagonal line directions, the electric fields are disturbed, and this might cause a problem in the quality of the accelerated beam. There is also a case where the beam collides with the electrodes, damaging and wearing the electrodes.
In order to solve these problems, an RFQ electrode as shown in FIG. 2 was invented. This RFQ electrode has a structure wherein a cavity is provided as a coolant channel inside a rectangular bar made of copper, or iron or alminium plated with copper, and a waveform with crest and trough portions is formed only on a surface facing the beam passage spacing. This RFQ electrode is the previous invention of the inventor of the present application. The crest and trough portions are provided only on one side of the electrode because an electric field wave is required only in the vicinity of the central axis of the beam. This allows the diameter of the coolant channel to be increased. Further, since a rectangular bar is adopted, the electrode is rigid in the height direction, and is resistant to bending. This electrode realizes ion beam acceleration with considerably high duty.
When a gap between electrodes is narrow, it is difficult to introduce an ion beam therebetween. The measure of how easily an ion beam is introduced between the electrodes is called acceptance. When the gap between the electrodes is, for example, 8 mm, it can be said that the acceptance is relatively large.
The following deals with height H of the electrode. The height H of the electrode is defined as the distance from a crest of one electrode to a bottom surface on the opposite side of the same electrode. In the case of RFQ electrodes to which a radio-frequency of 100 MHz is applied for acceleration of He.sup.+ ion, an electrode height H of 21 mm has been adopted conventionally. This is not without a problem. When the radio-frequency is increased, the cell length (.beta..lambda./2) is reduced, and accordingly optimum electrode height H is reduced proportionally. Thus, an optimum value of height H should be defined in relation to the frequency.
The RFQ electrode of FIG. 2 has a uniform waveform in the beam propagation direction, and is to be fixed to a supporting component (posts) by blazing. FIG. 3 shows a schematic arrangement of such an RFQ linear accelerator. On a position corresponding to four vertices of a square, electrodes A, B, C, and D are placed. Two kinds of posts are provided as vertical plates: One connected to the electrodes A and C, and one connected to the electrodes B and D. A long plate extending along the beam propagation direction, vertically supporting the posts, is a base. The electrodes, the posts, and the base constitute a radio-frequency resonance structure.
Although not shown, a coolant pipe is provided in the vicinity of the base and the posts. Also not shown are waveforms formed on the facing surfaces of the electrodes. In reality, the posts and the base are surrounded by a cylindrical vacuum chamber. Though it is desirable that the vacuum chamber is reduced as much as possible in view of the costs and space, a vacuum chamber that is too small lowers the power efficiency.
From a view of the above conventional problems, it is an object of the present invention to provide an RFQ accelerator, whose partial or entire configuration of RFQ electrodes is optimized, having desirable power efficiency, high acceptance for smooth introduction of an ion beam, superior mechanical strength, and desirable cooling efficiency, and to provide an ion implanter provided with such an RFQ accelerator.